Subsea multiphase pump technologies are presently in operation at several locations around the world. Two known technologies are helico-axial and twin screw pumps.
Helico-axial pumps are rotordynamic type pumps that have been developed specifically for multiphase pumping, and can handle flows of all-liquid or with high gas volume fraction without a reduction in capacity. A typical helico-axial stage consists of an axial flow impeller of helical blades followed by a diffuser to direct the flow to the next stage. The blade and vane geometries are designed to homogenize the gas-oil mixture to prevent separation while increasing the total pressure of the fluid.
In applications requiring a high pressure rise from the pump, helico-axial stages are typically utilized in a hybrid arrangement prior to centrifugal impeller stages. The pressure rise increase in the helico-axial stages reduces the gas volume of the fluid mixture to a level at which the centrifugal stages will operate adequately, typically less than 51 gas volume fraction. The bulk of the pump pressure rise then occurs in a series of centrifugal stages.
Gas volume reduction in a multiphase flow is essentially a reciprocal function of the pressure ratio referenced to the pump inlet pressure. Doubling the pressure reduces the gas volume by half. Pressure rise across a helico-axial pump stage is a constant differential pressure, typically a maximum of about 7 bar, regardless of inlet pressure. A helico-axial stage with a suction pressure of 7 bar can double the pressure ratio with a 7 bar pressure rise, decreasing the gas volume fraction by 50%. The same stage operating with a suction pressure of 70 bar can create a pressure ratio of 1101 with a 7 bar pressure rise, decreasing the gas volume fraction by 9%. The number of helico-axial stages in a hybrid pump has typically been limited to 7 due to rotordynamic limitations on the shaft length, limiting the maximum pressure rise to approximately 50 bar. This illustrates that the operating principles of helico-axial pumps limit the combination of suction pressure and gas volume fraction at which they can effectively operate. Subsea separators can operate at pressures that are greater than those at which helico-axial pumps can be effective. A helico-axial pump is described in U.S. Pat. No. 5,375,976, the disclosure of which is incorporated by reference herein.
Twin screw pumps are positive displacement type pumps, producing a constant volumetric flow rate in a progressing cavity formed between two interlocking helical screws on parallel shafts. The constant volumetric flow rate is determined by the volume of the cavity between the screws, the screw pitch, and the rotational speed. Tight clearances at the interfaces between the screw interlocking surfaces and between the screw tips and the housing are required to minimize recirculating flow that reduces the volumetric efficiency.
Because of their positive displacement operation, twin screw pumps provide an effective means of multiphase fluid transport. They can handle fluids with gas volume fractions as high as approximately 95% without a reduction in flow rate. For effective operation, a twin screw pump must handle fluids with higher viscosity (>200 cP) to create a seal at the small clearances between the screw surfaces and the housing. Lower viscosity fluids result in greater recirculating leakage flow that reduces the volumetric efficiency. Typically, subsea separators are more effective with low viscosity fluids, preventing the twin screw pump technology from being an attractive pumping option for subsea separation systems. A twin screw pump is described in US 2007/0274842, the disclosure of which is incorporated by reference herein.
A subsea separator is described in U.S. Pat. No. 5,526,684, the disclosure of which is incorporated by reference herein. Gas separator systems are described in U.S. Pat. Nos. 6,705,402; 5,207,810 and 4,886,530, the disclosure of which are incorporated by reference herein. Various types of inducers are shown in U.S. Pat. Nos. 3,339,821; 3,442,220; 6,435,829 and 7,207,767, the disclosures of which are incorporated by reference herein.